Add the ability to override tags in extended and included templates

[bcdickinson]

Is your proposal related to a problem?

Currently, if you have templates like this with an overridden template tag foo...

base.html:

<p>{% foo %}</p>

extended.html:

{% extends "base.html" %}

included.html:

{% include "base.html" %}

... it's not possible to control the output of foo from extended.yaml or included.yaml, only the values set in base.yaml will have any effect.

Describe the solution you'd like

It would be great if the ability to override tags cascaded/inherited the same way as template context.

Describe alternatives you've considered

It's not possible to work around this without duplicating the tag invocation in the inherited template, as far as I know.

[thibaudcolas]

Notes from today’s refinement session: still seems well worth solving, still seems very hard. Help wanted on this as it’s unclear how we should proceed.

Dupe (see comments): #8.